Brendan,
to IIS.
I think that 5" dob is a good choice for a first scope. The aperture is decent, and the compact package is small enough for your son to be able to move it from inside the house to outside without any help from you. He won't be able to do that with a 6" or 8" dob. That little scope can be plonked down on a table and away you go.
Such a scope will have plenty of aperture grunt to show you plenty without killing the piggy bank. My first "real" scope was a 5" SCT. Ok, not a 5" dob, but it was my only scope for many years, and I loved it and would have had the same visual experience as a 5" tabletop.
I WISH I had a 5" scope as a first scope! That 5" tabletop would have been blooming awesome! Mine was 2" Tasco refractor. I know I would have had a rough time of it negotiating a 6" or 8" dob by myself in and out of the house when I was 13, and even more so at 9. Even today with all the scopes I have at home (from 80mm to 17.5"), I still often use the 4" refractor and sketch with it (the pics below are a couple of sketches I've done with it of the Eta Carina, and of the area around the Coal Sack & Jewel Box cluster). So a 5" scope is not a whimp by any means. I have been SERIOUSLY thinking about getting a 5" Newtonian, even as a table top like you. I was only just talking to a good mate about this earlier today!
I have no problem saying a 5" dob is a great first scope for both you and your son
Alex.